Percutaneous electrodes were introduced in the 1970's as a minimally invasive technique to deliver spinal cord stimulation (SCS), so as to screen patients for satisfactory responses before implanting permanent electrodes via laminectomy. The technique was quickly adapted for chronic implantation, and since the 1980's the majority of permanent systems have used percutaneous rather than laminectomy electrodes.
Percutaneous SCS electrodes are inserted into the epidural space through a Tuohy type needle, bent at its tip to allow the electrode to emerge at an angle and then ascend cephalad, in or parallel to the midline. Since the early 1980's they have had multiple contacts, forming a linear electrode array. Two or more can be placed in parallel to form a two-dimensional array. Insertion is guided by intraoperative fluoroscopy and by test stimulation as the electrode is advanced in the epidural space. The tip of the advancing electrode is bent at an angle so that it may be steered as it advances by rotating it, moving the tip from right to left. Right-left positioning is critical, to such an extent that a well-placed midline electrode placed percutaneously was seen to outperform two electrodes placed side by side, in a series of controlled trials performed at Johns Hopkins Hospital from 2003-2005. In engineering terms, the system (patient and stimulator) resolves smaller dimensions than the width of standard percutaneous electrodes, such that simply placing more electrode contacts does not substitute for precise placement.
Anterior-posterior position (in front of or behind dorsal epidural fat) is important, as well—not only because proximity of the electrodes to the spinal cord increases efficiency and selectivity (of dorsal columns vs. dorsal roots), but also because recruitment of small pain fibers in ligamentum flavum (behind epidural fat) commonly causes painful side effects, and one way to mitigate them is to keep the electrode in front of epidural fat and in direct contact with the dura. An experienced operator will attempt to keep track of which way the tip of the advancing electrode is pointing, even on monoplanar (anterior-posterior) fluoroscopy, and attempt to direct the tip of the electrode from right to left and from front to back.
Where SCS is to be administered by less experienced practitioners, multiple electrodes will often be used to bracket or “carpet bomb” the target area. Most (70-80%) percutaneous systems currently implanted have dual electrodes—in part for this reason, and in part to provide redundancy—the demonstrable inferiority of dual lead systems for common low back conditions notwithstanding. The best use of this technology may in fact be to place triple electrodes, which offer potential advantages whether they are placed perfectly, or by experienced hands (taking advantage of transverse tripole capabilities) or imperfectly (simply offering redundancy).
In any of the above settings, and whether in experienced or inexperienced hands, percutaneous SCS electrode steerability is very important, and none of the presently available products address it adequately. Not only must the electrode be steerable, but the steering must also be variable. More particularly, major steering input is required as the electrode emerges from the tip of a Tuohy or similar needle, which necessarily is not parallel to the epidural space as seen in the sagittal plane. The electrode must negotiate a bend; if it does not bend sufficiently, it may indent the dura (or even the spinal cord). If it deviates to one side, it will still tend to continue ventrally, into the “gutter” in the lateral epidural space. Not only is there a bend in the sagittal plane, there is often one in the coronal plane, as the needle generally is not parallel to the midline, as it is inserted off the midline to avoid the spinous processes. Thus, as the electrode emerges it tends to deviate laterally, to one side or the other, and once it has done so it may not be possible to steer it back to the midline. An analogy may be made with descending a steep “on ramp” onto an icy highway which has a high crown in the center of the road and deep gutters on either side.
Once having deviated substantially from the midline, even if it is steered back, as seen on A-P fluoroscopy, the electrode may end up ventral to the spinal cord and dura, and this may not be appreciated without changing to a lateral fluoroscopic view.
Minor steering inputs are required after the electrode has negotiated the bend at the tip of the Tuohy needle and is ascending in the midline. The large bend which was an asset in negotiating the initial turn becomes a liability, as relatively small right-left adjustments are made as the electrode is advanced to its final position. Pointing a large bend to the left or right causes major unwanted deviations to either side; pointing it dorsally tends to engage dorsal ligamentous structures and webs, at the same time moving it away from the spinal cord. Pointing it ventrally, so that it slides along the dura, pushes the dura anteriorly; this tends to be unstable, and the electrode tip may rotate and jump to one side or the other. Furthermore, when the electrode is curved significantly in the sagittal plane its contacts are at different distances from the spinal cord, and stimulation thresholds vary substantially. This frustrates test stimulation to guide placement.
Since they were first introduced in the 1970's, all percutaneous SCS electrodes have consisted of rigid cylindrical metal contacts on flexible catheters (for example silicone elastomer, polyethylene, and more recently polyurethane). The flexible catheter, within which are electrical conductor(s) to the electrode contact(s), accommodates bending, allowing for example insertion through a Tuohy needle with a bend at the tip, so that the advancing assembly can negotiate the bend. The rigid contacts must negotiate the bend in the needle first, and as they increase in length it becomes all the more difficult to negotiate this bend, and then to steer to the target beyond.
As shown in the top view FIG. 1, a typical current, commercially available percutaneous electrode assembly (“lead”) 10 has a removable, malleable stylet 20, which may be inserted within the body of the flexible, hollow lead 10. A bend in the stylet 20 causes the lead 10 around it to bend correspondingly. The stylet 20 typically is bent at the tip—indeed, most models come with a bend already in place—and typically the stylet may be inserted all the way to the tip of the lead 10, and to the most distal electrode contact, so that the tip of the lead 10 and the electrode contacts assume a bend. The distal end of the lead 10 has a plurality of rigid electrode contacts 12, and flexible spacer portions 13 between the contacts 12. As the bent tip is advanced under fluoroscopic guidance it can be steered by rotating the stylet 20 and/or the lead 10 so that the advancing tip deviates to the left and the right, dorsally and ventrally, etc. As the bent tip is advanced, as seen on A-P fluoroscopy, rotating it allows it to be steered. Not only can it be steered right-left, it can also be steered dorsal-ventral if the operator follows it on lateral as well as A-P fluoroscopy (and/or keeps mental track of its orientation using the A-P view alone). The electrode tip is most easily steered if it responds in a linear fashion to rotation by the operator at the other end of the lead. Further, as shown in the bottom view of FIG. 1, the bend in stylet 20 may be advanced into and withdrawn from lead 10 so as to modify the extent to which lead 10 bends. If the stylet 20 is not inserted fully within the lumen of lead 10, the bend of stylet 20 is located proximal to the distal end of lead 10, resulting in a larger bend of lead 10 while the tip of lead 10 lacks stiffening and, thus, remains flexible.
Most stylets are coated with Teflon, which facilitates removal and replacement, as is often necessary to change the bend at the tip. It is typically necessary to increase the bend to negotiate the sharp turn at the tip of the needle, as the tip of the electrode enters the epidural space and must be steered to (or back to) the midline; once this has been achieved, a large bend becomes a liability, as noted. The only way to reduce the bend is to withdraw the stylet and replace it—after straightening it, or substituting another stylet with a smaller bend. This is cumbersome, and it can be challenging for the operator to thread the stylet back into the tiny lumen of the lead. Some leads resist this maneuver, and on occasion it becomes necessary to ask for a new lead—which means withdrawing the existing lead and giving up the position achieved so far.
Electrode steerability is best if the curved electrode tip is seen on fluoro and felt by the operator's hand to move in a linear 1:1 fashion as the stylet emerging from the end of the lead assembly is rotated by the operator's hand. A right-handed operator will generally rotate the stylet (which may rotate within the lead) or the stylet/lead assembly (which may rotate together) with his/her left hand, as the thumb and index fingertip of the right hand advance and withdraw the body of the lead as it emerges from the hub of the Tuohy needle in the epidural space. Some Seldinger wire lead blanks achieve this, but no electrode assembly does so; to varying degrees they lag, jump, and sometimes seem to wind up, as though lacking in torsional rigidity.
The malleable stylet may rotate in unison with the “lead” around it, or it may rotate within the lead, or it may (and commonly does, to varying degrees) do both. One presently available device may be clipped to the end of the lead so that the two necessarily rotate together (at least within and adjacent to the clip; they may decouple farther away.) This incurs friction between the outside of the lead, along its length within the needle and the patient; the friction is cumulative as the lead is advanced, and ultimately the stylet begins to rotate within the lead, thus impeding steerability. Rotation and translation of the tip no longer track operator inputs 1:1.
Other presently available devices may have a knob at the end of the stylet, but no clip secures it to the lead; the stylet is unconstrained and can rotate within the lead. The tip of the stylet may engage a bearing within the lead tip in order to reduce friction. As the length of the lead and stylet assembly increases, however, the cumulative drag or friction between the outside of the stylet and the inside of the lead body can become significant, so that the lead tip no longer tracks operator inputs faithfully, 1:1.
Other problems and idiosyncrasies are introduced by assembly and packaging of some available devices. For instance, if the metal stylet inside the plastic lead body is bent, then the plastic lead tends to take a “set” which remains, if only temporarily, after withdrawal of the stylet. This can be useful during implantation, in that it provides finer control over steering, but it then becomes a liability: the lead retains this bend following implantation, and so it may curve left-right, or dorsal-ventral. With time, the curve may straighten out, with unpredictable results.
Moreover, the “set” in the plastic and the bend in the stylet are, of course, pointing in the same direction when the assembly is removed from the package, and they remain so unless disturbed. It is very common, however, to withdraw the stylet at least in part during placement, and if this is done it may not be in phase with the lead, i.e. pointing in the same direction, when the stylet is re-inserted. Sometimes the stylet is removed and replaced, e.g. with a stiffer one, or one with a different bend. In To the inventor's knowledge, there is no provision to address this in any existing product. Marking the stylet and the lead in order to allow alignment would be possible, but they may not be rigid in torsion, and so this might not be reliable.
Still further, the “set” in the plastic lead is, on balance, a liability. Current potential solutions include preassembling with a straight stylet, and allowing the user bend it as desired, or furnishing the stylet separately. Presently available packages include leads that are shipped with a bent stylet in place, incurring the above problem.